A new study shows that the 10-year-old chicken pox vaccine is highly effective. It finds that a relatively small percentage of vaccinated children still get the disease, but the cases are mild and much less contagious to other youngsters, even in homes with lots of close contact.

A young California boy named Connor Nichols loves to play with his little toddler sister Kate. In fact, this big brother doesn't even mind kissing her.

The children's mother, Jeannette Nichols, says the pair is very close. Naturally, she worries about the health implications when Kate drinks from Connor's cup or picks up his fork or toothbrush.

"They're constantly touching and I'm sure the germs are spreading back and forth," Ms. Nichols said.

It is hard to protect children from each other's germs, but immunization can help. A study in The Journal of the American Medical Association shows that the chicken pox vaccine introduced in the mid-1990s is highly effective in protecting youngsters against disease, even in households where illness spreads easily.

The lead author of the study is public health expert Jane Seward of the U.S. government's Centers for Disease Control.

"If vaccinated children come into close contact with a chicken pox case, four out of five of those children won't get any chicken pox, and one out of five may get a very mild form of chicken pox that won't have them be nearly as sick as if they'd been unvaccinated," Ms. Seward said.

Ms. Seward and her colleagues analyzed chicken pox data gathered between 1997 and 2002 in an area called Antelope Valley, a California community of 300,000 people near Los Angeles. The researchers were especially interested in the vaccine's impact on contagiousness among children in close contact. Ms. Seward says the results show that vaccinated children who got the disease were only half as contagious as unvaccinated children who got sick.

Although chickenpox is not considered an important public health problem, the World Health Organization says almost every child or young adult around the globe experiences it because it is extremely contagious, so the social and economic consequences off loss of school or work time should not be underestimated. Before the vaccine was available, the ailment once killed 100 Americans each year.

The agency notes that vaccination is the only measure likely to control chicken pox because drugs are very expensive and normally used for treatment after exposure. A recent analysis in the United States shows that routine chickenpox vaccination is likely to save five times the investment.

Similar studies are not available from developing countries, and the WHO concedes that the disease is less likely to be addressed where there is a heavier burden of more serious illness like AIDS, tuberculosis, and malaria. But it also notes that the public health impact of chickenpox might be increasing in regions with high rates of HIV.

Near Los Angeles, Jeannette Nichols plans to have her children, Connor and Kate, immunized.